In many modern communications systems, the information to be transmitted is coded both in the phase and in the amplitude of a carrier signal. The combined coding of the information makes it possible to achieve considerably higher data transmission rates than in conventional types of modulation which use pure amplitude or phase modulation. Types of modulation which use pure phase modulation are referred to as types of modulation with a constant envelope (constant envelope modulation). One example of such a type of modulation is so-called frequency shift keying (FSK, GFSK) in which the information to be transmitted is coded in the form of a frequency hop or a phase shift. Examples of types of modulation using phase and amplitude modulation are, in particular, PSK (phase shift keying) modulation such as Π/4-DQPSK, 8-DPSK or 8-PSK modulation and quadrature amplitude modulation (QAM).
In contrast to analog amplitude or frequency modulation, said types of modulation are also referred to as digital types of modulation or types of modulation with a non-constant envelope. The types of modulation with a non-constant envelope are principally used in modern communications standards such as Bluetooth HDT (high data rate), GSM/EDGE, UMTS/WCDMA or WLAN.
FIG. 9 shows a constellation diagram for illustrating the transmission of data using the QPSK type of modulation. In this case, the x-axis represents a first real component I which is also referred to as an in-phase component. The y-axis forms a second complex conjugate component Q, the so-called quadrature component. Depending on its content, the information to be transmitted is coded by a value pair i, q at one of the points illustrated. Such a value pair i, q is referred to as a symbol. In the example, a symbol thus represents two bits of data content in the case of QPSK modulation. Depending on the data content to be coded, for example the bit sequence 01 11 10, the amplitudes and phases of the i and q values change over time.
In addition to a symbol being represented by a value pair i, q, it is also possible to specify the symbol in terms of its phase Φ and its amplitude r, the polar representation of the symbol. Both representations written using I/Q and r/Φ are synonymous.
In order to transmit and modulate the information onto a carrier signal, use is made, inter alia, of an I/Q modulator. FIG. 1 shows a modulator known to the inventor. In this case, the symbols which are to be transmitted and are in the form of digital in-phase and quadrature values are first converted into analog signals using a digital/analog converter and are then filtered using a low-pass filter. The continuous time values i(t), q(t) are then supplied to a respective mixer. Two signals having a phase offset of 90° with respect to one another are used as local oscillator signals, as illustrated here. Following frequency conversion by the two mixers, the converted signals are added, and amplified to the output power using the power amplifier PA, and are then output via the antenna.
When designing a transmitter output stage having the I/Q modulator illustrated, it must be ensured that there is a particularly accurate phase difference of 90° between the two local oscillator signals supplied. DC signal components in the supplied signals i(t) and q(t) are likewise converted to the transmission frequency by the mixers and may thus result in errors in the transmission of data. The illustrated I/Q modulator also consumes a relatively large amount of power on account of the two mixers.